A sectional view of a gas turbine engine 10 of the turbo fan type is shown in FIG. 1. While not shown, the engine 10 may also be a geared turbofan engine. The engine 10 includes a nacelle 11 that surrounds the fan 12. The fan 12 may be connected to a nose 13 for aerodynamic purposes and, as the fan 12 rotates, it generates two primary air flows including a bypass airflow indicated by the arrow 14 and that flows between the nacelle 11 and the engine case 15 which serves as a housing for the primary engine components. The fan 12 also helps to generate an air flow that passes through the engine case 15 as indicated by the arrow 16. A low-pressure compressor 17 is disposed aft of the fan 12 and is coupled to a low-pressure turbine 18 by the shaft 21. A high-pressure compressor 22 is disposed aft of the low-pressure compressor 17 and forward of the combustor 23. The combustor 23 is disposed between the high-pressure compressor 22 and the high-pressure turbine 24. The high-pressure turbine 24 and the high-pressure compressor 22 are coupled together by the outer shaft 25.
Air enters the engine 10 via the fan 12 before being compressed by the low-pressure compressor 17 and before being compressed again by the high-pressure compressor 22 before being combusted in the combustor 23. The combustion gasses then rotate the high-pressure turbine 24 and low-pressure turbine 18 (which drive the high-pressure compressor 22 and low-pressure compressor 17 respectively) before exiting the engine 10 through the nozzle 26.
One primary problem associated with gas turbine engines 10 is leakage, especially in the compressors 17, 22. For example, a high-pressure compressor 22 is illustrated in FIG. 2. The high-pressure compressor 22 is surrounded by the case 15 and includes a plurality of rotors 27-32. Each rotor 27-32 includes a disk 33-38. Each disk 33-38 may include a radially inwardly end 39-44 which may serve as a hub for connecting the disks 33-38 to the shaft 25 (see FIG. 1). Each disk 33-38 may also include a radially outward end 45-50. The radially outward ends 45-50 are typically coupled to rotor blades 51-56. The rotor blades include distal tips 57-62 which are disclosed in close proximity to interior surfaces of the case 15. The clearance between the rotor blade tips 57-62 and the interior surfaces of the case 15 are one source of leakage in such a high-pressure compressor. Another source of leakage involves the stators 63-67 which are coupled to the case 15 as well as shrouds 68-72. As shown in FIG. 2, the shrouds 68-72 may be of a fixed dimension (see the shrouds 70, 71 and 72) or the shrouds may be variable (see the shrouds 68, 69). Either way, clearance between the variable stators 63, 64 and the rotors 27, 28 and clearance between the non-variable stators 65-67 and the rotors 29-32 is another source of leakage.
The compressor 22 of FIG. 2 includes stators 63-67 of the shrouded type. Turning to FIG. 3, another high-pressure compressor 122 is disclosed with “cantilevered” stator 163-167. The reader will note that the cantilevered stators 163-167 are secured to the case 115 and are not physically connected to nor engaging the rotors 127-131. Instead, there is clearance between the radially inwardly directed tips 76-79 and the radially outward ends 145, 147, 149 of the rotors 127, 129, 131. The clearances between the tips 76-79 and the radially outward ends 145, 147, 149 of the rotors 127, 129, 131 are another source of leakage for the compressor 122.
The negative impacts of such leakage between rotor blades and the case or between stator vanes and the hubs are well documented. An increase in the rotor tip clearance leakage leads to a reduction in stage pressure rise, efficiency and flow range. For example, rotor blades with normal operating clearances generate a 1.5% reduction in efficiency for every 1% increase in the clearance to blade-height ratio. Such leakage also increases the stall margin. Similar effects are caused by leakage at the stator vane tip, whether the stator vanes are of the shrouded type as shown in FIG. 2 or of the cantilevered type shown in FIG. 3. Another option that is not shown in FIG. 2 or 3 is to employ an abradable rub system which is expensive, prone to wear or deterioration and therefore results in a loss in effectiveness.
Therefore, there is a need for an improved stator vane design which minimizes leakage and forces more air flow to pass across the body of the stator vane.